1. Field of the Invention
The present invention relates to a mask used for a lithography process to manufacture a semiconductor device, and more particularly to a mask having auxiliary patterns and a mask data generation method for such a mask.
2. Description of the Related Art
As semiconductor devices have been highly integrated, device patterns have been enhanced in fineness. For example, a dynamic random access memory (DRAM) having a minimum dimension not more than 100 nm and a capacity of 1 gigabit has practically been used. Optical lithography technology serves as an engine of this enhancement of fineness.
In the field of optical lithography, application of recent super-resolution technology has allowed formation of a fine pattern having a size not more than a half of a wavelength of light. Particularly, in a dense pattern such as a line-and-space pattern, in which lines and spaces are repeated at constant pitches, a sufficient depth of focus can be obtained by an off-axis illumination method. It is noted here that the line-and-space pattern is hereinafter referred to as an L/S pattern. In the off-axis illumination method, a normal-incidence component is cut off from mask illumination light so that a mask is illuminated with oblique-incidence light. In a normal image formation, three beams of a zeroth-order diffracted light beam, a positive first-order diffracted light beam, and a negative first-order diffracted light beam are condensed from a mask pattern by a projection lens (image formation of three-beam interference).
In contrast to the normal image formation, in another off-axis illumination, one of positive and negative first-order diffracted light beams is discarded (or cut off so that it does not enter a projection lens). Thus, an image is formed by two beams of a zeroth-order light beam and one of the positive and negative first-order diffracted light beams (image formation of two-beam interference). When images formed by the three-beam interference and the two-beam interference are compared in their best focused conditions, a contrast of the image formed by the two-beam interference is lower than that of the image formed by the three-beam interference because one of the positive and negative first-order diffracted light beams has been discarded. However, when an angle of incidence on an image formation surface (semiconductor substrate) is taken into consideration, the image formed by the two-beam interference has an angle of incidence that is a half of an angle of incidence of the image formed by the three-beam interference. Accordingly, the two-beam interference has a smaller degree of defocus when an image is out of focus. Thus, the two-beam interference can obtain a light intensity distribution that is sufficient to form a resist pattern with a wide focus range.
It has been known that a depth of focus (a focus range in which a resist pattern can be obtained) can be increased by the use of a half-tone phase shift mask. In the half-tone phase shift mask, a mask pattern being a light shield region is formed as a semitransparent region so that 2% to 20% of light leaks. The phases of the leaked light and the light passing through the peripheral transparent region are inversed by 180°. In a case of a line-and-space pattern that produces diffracted light, the balance between the zeroth-order diffracted light and the positive (or negative) first-order diffracted light can be improved by the use of a half-tone mask and off-axis illumination. As a consequence, the contrast can be increased.
However, the off-axis illumination method is less effective in an isolated pattern, which produces no diffracted light. Accordingly, a depth of focus is not largely increased. Conversely, reduction of NA or σ is more effective in order to increase a depth of focus in an isolated pattern. Reduction of NA means that a mask is illuminated with light close to vertical components. In case where a half-tone phase shift mask is used, reduction of σ is more effective in improvement of a depth of focus. These conditions to increase a depth of focus in an isolated pattern result in a lowered resolution of a dense pattern. Accordingly, it has been difficult to maintain exposure characteristics of both a dense fine pattern and an isolated pattern. Under theses circumstances, a method using a fine pattern called an auxiliary pattern, which does not serve to resolve an image, has been developed to obtain a depth of focus in both a dense pattern and an isolated pattern.
For example, Japanese laid-open patent publication No. 4-268714, which is hereinafter referred to as Patent Document 1, discloses auxiliary patterns (page 3; FIGS. 4(a) and 4(b)). According to Patent Document 1, when fine hole patterns and slit patterns are to be formed by off-axis illumination, a depth of focus can be increased by arranging auxiliary patterns so that the hole patterns and slit patterns approach a periodic pattern. Furthermore, Patent Document 1 discloses that the same effects can also be obtained in line patterns. When a mask including auxiliary patterns is used under off-axis illumination conditions, an image is nearly formed by two-beam interference so that a depth of focus is increased. Upon arranging auxiliary patterns, the position and dimension of the auxiliary patterns give an influence on a depth of focus in a device pattern. An optimum value of a space between an auxiliary pattern and a main pattern varies depending upon their dimensions and applied optical conditions. However, an appropriate range of a space between an auxiliary pattern and a main pattern is from a dimension corresponding to a limit of resolution of optical conditions (½ of a pitch of an L/S pattern to be formed) to about 1.5 times that dimension.
Moreover, Japanese laid-open patent publication No. 5-002261, which is hereinafter referred to as Patent Document 2, discloses a combination of off-axis illumination and a half-tone phase shift mask to improve reduction of a contrast in image formation of two-beam interference (page 3; FIG. 1). In the image formation of two-beam interference, one of positive and negative light beams is cut off. Accordingly, the zeroth-order diffracted light, which has information on average brightness, becomes excessively strong relative to the positive (or negative) first-order diffracted light, which has information on pitches. Therefore, an amplitude of peak/bottom is smaller than an average value in a light intensity distribution. Thus, a contrast of the light intensity is lowered. By using a half-tone phase shift mask, intensity of the zeroth-order diffracted light can be reduced. In this manner, intensity of the zeroth-order light and the positive (or negative) first-order light can properly be adjusted so as to improve a contrast of the light intensity.
The following patent documents describe technology to further improve the resolution. Japanese laid-open patent publication No. 3-071133, which is hereinafter referred to as Patent Document 3, discloses auxiliary patterns of phase shift layers disposed adjacent to isolated patterns. Japanese laid-open patent publication No. 6-242594, which is hereinafter referred to as Patent Document 4, discloses that auxiliary patterns each having a width SW not more than a minimum line width P/2 are arranged at distances S from isolated patterns so that a relationship of P/2<S<(P-SW) is satisfied. Japanese laid-open patent publication No. 2004-348118, which is hereinafter referred to as Patent Document 5, discloses auxiliary patterns corresponding to hole patterns. However, both of these auxiliary patterns and optical proximity correction (OPC) are used in the current optical lithography for achieving a high level of fineness. Accordingly, it has been desired to optimize both of auxiliary patterns and OPC in order to provide fine patterns.